of the Skull and the Skeleton. 361 
So far, then, in its general plan the skull follows the vertebral 
type. But by the narrowing down of the bronchial tube, and 
the resistance of the surrounding organs, the mouth below and 
the brain-case behind, a powerful ossifying force, of which we 
have already seen evidence in the trachea, comes into play, 
different to that of the chest; for there the digestive canal is 
enclosed by the breathing-apparatus, while here the breathing- 
canal is small, and nearly shut in by the digestive canal below 
and by the resisting vertebral centrum above. So that, seeing 
what the result of the thoracic action was in the development of 
ribs and in the development of the trachea, it must be antici- 
pated that ossifications will likewise take place in the skull from 
the same cause in the direction of greatest resistance, 7.e. above 
and below the termination of the trachea in the skull; and accord- 
ingly we find a triple series of bones above and in front, and 
another triple series below and behind. The first series consists 
of the nasal bones, the ethmoid, and the vomer, the nasal bones 
and vomer being in the position of epiphyses of the ethmoid ; 
and below these are the pterygoid and palatine bones, and an 
unossified blastemous extension of the latter anteriorly, on which 
the maxillary and premaxillary bones are developed, just as the 
prehensile bones of the lower jaw were developed on a cartila- 
ginous extension. ‘This, then, is clearly a distinct region of the 
skull, to which there is obviously nothing even analogous in a 
vertebra; and in reviewing its comparative osteology, I find no 
reason for considering it less a fundamental essential of a deve- 
loped skull than the neural region itself. And just as the brain- 
case is known as the neural region, so this part may well be 
called the bronchial region ; for just as the former is a modified 
neural arch and its centrum, so the latter is a modified termina- 
tion of the trachea: and thus, although the skull appears in this 
matter to deviate from our conception of a vertebra as merely 
an ossified structure, yet it conforms even in that deviation to 
the plan of a segment of the body, and so brings the skeleton 
into a closer and more natural unity. 
The lower jaw and its upper appendages being a modified rib, 
we thus exhaust all the vertebral elements without accounting 
for the maxillary or premaxillary, or the distal elements of the 
lower jaw exterior to Meckel’s cartilage. The maxillaries, by 
development no less than by function, are the anterior epiphyses 
of the palatines ; while the premaxillaries appear to be the lateral 
epiphyses of the ethmoid. Such is the circumstance of their 
origin, though no doubt their development is due to the same 
pressure by which we have seen that all bones are formed. Thus 
in the elephant, where the premaxillaries have to support the 
enormous tusks, they attain an enormous development, covering 
Ann. & Mag. N. Hist. Ser. 3. Vol. xvii. 25 
